In this CL, we add a set of checks that automatically run
against a new GitHub PR after it is imported into Gerrit
in order to flag common issues, such as missing a bug
reference or ending the first line of the commit with
a period. See below for the current set of defined rules.

The intent is to save maintainer time, as well as
perhaps make a better initial experience for a
new contributor via faster feedback with potentially
helpful linked resources.

By attempting to engage a new contributor as soon as possible
along with some pointers, we also hope to get them
to reply (successfully) in Gerrit, which in turn helps
a potential reviewer see that the contributor knows
how to use Gerrit.

The package comment in rules.go provides an overview
of the implementation approach.

The rules themselves are defined as short functions
in rules.go.

We currently have 100% test coverage within the rules package.

Sample results from running on a corpus of 1000 GitHub PRs
that were imported into Gerrit:

  Avg. findings per CL: 1.42
  CLs with findings:    74.5%

  CL hit %   Finding
  --------   ------------------------------------------------
      9.9%   title: no package found
      0.1%   title: no colon then single space after package
     13.0%   title: no lowercase word after a first colon
      2.0%   title: ends with period
     20.8%   body: short
      6.1%   body: no sentence candidates found
     21.7%   body: long lines
     10.5%   body: might use markdown
      2.2%   body: still contains PR instructions
      0.9%   body: contains Signed-off-by
     43.9%   body: no bug reference candidate found
      9.3%   body: bug format looks incorrect
      1.5%   body: no bug reference candidate at end

See golang/go#61573 for additional background.

Updates golang/go#61316
Fixes golang/go#61573

Change-Id: I866cf650608d6ce9c6783aabc17a219f1815908c
Reviewed-on: https://go-review.googlesource.com/c/build/+/513397
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: t hepudds <thepudds1460@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>